An ink jet recording method has hitherto been used as a recording method that ejects ink droplets through a print head or a spray nozzle at a high speed toward a recording material in response to electric signals sent from a computer or the like to perform printing on an ink-receptive layer in the recording material. By virtue of small noise and high-speed printing, the ink jet recording method has become widely spread, for example, in the fields of copying, facsimile, poster, and display. Properties required of inks used in the ink jet recording method include no change in properties with the elapse of time, stable retention of satisfactory density of an image formed using the ink for a long period of time, and excellent original reproduction of an image printed on a recording material.
To satisfy these property requirements, the ink used should not clog a head nozzle and a filter provided between an ink cartridge and the head. Therefore, for the ink, ink properties such as density, viscosity, particle diameter, and concentration should be regulated. The ink is generally prepared by mixing a colorant such as a dye or a pigment with a dispersant for dispersing the colorant, a solvent and optionally an additive.
In particular, when a white ink for ink jet recording is printed on a black or transparent recording material having low brightness, a highly visible printed matter can be provided. The white ink is also effective for marking of various products. When a color ink other than the white ink is printed on the recording material, sharp printing cannot be reproduced due to the influence of a color of a substrate. In order to overcome this problem, a method is adopted in which, after printing, underlying printing is performed with a white ink having a satisfactory substrate opacifying property to opacify the substrate in its area remaining unprinted, and other color ink is then printed on the white background. The white ink generally uses a titanium oxide pigment as a colorant. The conventional white ink using a titanium oxide pigment causes sedimentation/coagulation of the titanium oxide pigment with the elapse of time during storage due to, for example, the specific gravity of pigment particles and a solvent used, particularly when an alcohol solvent is used, often resulting in lowered redispersibility of the titanium oxide pigment.
In order to solve the above problems of the prior art, Japanese Patent Application Laid-Open No. 107964/1994 (patent document 1) proposes a white pigment composition into which a specific compound has been mixed. Specifically, the white pigment composition is produced by dispersing a titanium oxide pigment subjected to only alumina treatment in an alcohol solvent using a carboxyl group-containing acrylic resin as a dispersant. The white pigment composition disclosed in patent document 1 has been proposed with a view to improving the redispersibility of the precipitated white pigment and thus is unsuitable in an actual work (an ink jet printing work) where there is a demand for the use of the white pigment composition as such is used without redispersion of the pigment settled in the ink. Further, when the titanium oxide pigment is used as a white pigment, during storage of the ink, the pigment contained in the ink is likely to coagulate and settle. Among others, when the white pigment composition is filled as ink into an ink jet recording apparatus and is not used in this state for a long period of time, the ejection stability of the ink is poor when printing is restarted. Further, in this case, the ink clogs the printer nozzle, and, in the worst case, the printer head should be replaced.
On the other hand, as compared with a low-boiling point solvent which has been commonly used as a solvent for ink compositions, solvents of glycol ether dialkyl ether compounds have a high boiling point and a low vapor pressure, emit no significant odor, are highly safe, and have an excellent work environmental property. The glycol ether dialkyl ether compound solvent, however, has a lower resin solubility than the low-boiling point solvents which have hitherto been used, and, thus, the number of types of commercially available solid resins usable in this case is limited. Accordingly, the use of resins having a high glass transition temperature (Tg) such as methyl methacrylate homopolymers is generally difficult. The use of such acrylic resins poses problems of storage stability and ejection stability of the ink. Further, for acrylic resins of which the solubility has been improved by mixing an acrylic monomer having the effect of increasing the solubility to lower the Tg value and to lower the molecular weight, satisfactory print drying properties cannot be realized without difficulties. Further, when the acrylic resin, the titanium oxide pigment and the dispersant are incompatible with each other in mutual solubility, coagulation sedimentation of the pigment in the ink is likely to proceed during storage of the ink and, consequently, the ink has suffered from problems of storage stability and ejection stability.
Accordingly, at the present time, the development of oil-based white ink compositions for ink jet recording, which are excellent in solubility (dispersibility) between ingredients of the ink composition and long-term storage stability and ejection stability of the ink compositions and can realize good print drying properties, has been urgently demanded.